My attempts to make a rapid prototyping machine that I will use to make parts for a machine that will be able to make parts for a copy of itself.

Tuesday, 3 November 2009

Hacking with Erik

Erik de Bruijn (RepRap evangelist) is in the UK at the moment visiting Salford and Nottingham universities to spread the word. Yesterday he came here to see HydraRaptor. We spent a very interesting afternoon and evening, swapping extruder ideas, comparing objects we had made, and doing a couple of very successful experiments.

The first was something I had been wanting to try for a long time, and that was reversing the extruder drive to stop ooze. My latest extruder (details to follow) has a much smaller melt chamber but still has significant ooze when extruding PLA. Erik is pursuing the Bowden extruder idea, which should benefit even more from reversing.

Because my machine is controlled by Python, rather than g-code, it is very easy to try out things like this. We hacked the code to instantaneously reverse for a short distance very quickly at the end of each filament run. After moving to the start of the next run it fast forwards the same distance that it reversed before resuming the normal flow rate.

I designed a simple test shape to allow the results to be compared. It is a 15mm square with four 5mm towers at each corner. I am not using Enrique's latest Skeinforge which I think would minimise the extruder moves in fresh air to just three per layer. This is with a very old version that does the four outlines and then returns to fill each of them in.

Plenty of hairy bits showing the ooze. These can be removed easily, but what is worse is the object will be missing that amount of plastic making it weaker. This can be extreme with a thin structure which is remote from other parts of the same object.

We tried reversing 1 mm at 8 times the extrusion speed to start with. That worked but was obviously more than was needed. We tried 0.25mm which was too little and settled on 0.5mm, although a lot of that is taken up by the motor bracket flexing. I need to make it stronger.

The result was no hair at all!

A very simple fix for a problem that has used a lot of my time in the last two years.

The second experiment was something Erik wanted to try. He has discovered that PLA is soluble in caustic soda, so potentially could be used as soluble support material for ABS. The question was: can we extrude ABS onto PLA and get it to stick well enough to resist warping?

We made a 5mm thick slab of PLA 20mm wide and 40mm long, 90% fill. On top of that we extruded a 30 x 10 x 20mm block of ABS with a 25% fill.

The ABS looks very glossy so I think it may have some PLA in it. Possibly we needed to flush it through for longer. The ABS block is also a bit scrappy. The reason was that the extruder was playing up. It was leaking plastic, hence the burnt bits and the stepper motor was skipping steps leaving a deficit of plastic. This extruder had never done ABS before and still has some teething problems, but it shows that ABS will bond to PLA well enough to stop it curling.

Next we extruded a block of PLA on top of the ABS.

That also bonded well. The messy bit at the join is because HydraRaptor did its normal circuit of the object that it normally does on the first layer but it was in mid air.

To see how well they were bonded we put the PLA base in a vice and attached a small g-clamp to the PLA block on top. The g-clamp was pulled with a strain gauge until the ABS came way from the base at about 8Kg. Interestingly the first layer outline of the ABS was left on the PLA. That was deposited at 215°C whereas the infill of the first layer was at 195°C. These are the values I use for depositing ABS onto a raft, so in an object layer on top of support it would be 240°C giving a stronger bond. See Erik's writeup and video here.

So PLA looks like a good candidate for supporting ABS. They bond well and PLA is very rigid to resist warping. It can be dissolved with drain cleaner but also I expect it would be easy to peel when softened in hot water.

24 comments:

I'm excited to continue experimenting with two extruders and use PLA as a support for a more complicated shape.That is, if I'm allowed to take my extruder back across the border. It does look 'interesting' and might pose a problem since you don't really explain very easily what it is.

@Evan, I will need a second extruder close to the first or swapped by an automatic head changer. Erik plans to use the Bowden idea to have several nozzles side by side. I will wait to see how well that works, and how the Mendel head changing turns out before persuing this further.

@Freds, I would have to do that to make the machine work standalone, but currrently the PC waits for the extruder to reverse or fast forward before telling the table to move. It all works accurately because the link from the PC is 100Mb and the link to the extruder 100Kb.

Currently the motion controller knows nothing about the extruder controller except how to forward messages from the Ethenet to the I2C. I like it that way so I can do all the experimenting in Python on the PC and rarely need to change the firmware.

I have tried the reversing idea and it worked well stopping the ooze but increased the number of jams that I had. I reversed as fast as I could make the stepper move for 1500 pulses and then on starting made it extrude fast for 2000 pulses as plastic seems do evaporate especially after a longer stop.I found flexing was also a problem and on short stops I ended up with a blobby start- unsurprisingly but my programming skills are not up to differentiating the difference. I am still woking on making the extruder work reliablymy MTBF is currently only about 2 hours which is really anoying- Anyway great work as usual nophead.

I think Erik will have to answer those questions. I think he has tried it.

@Andy (Hall I presume),The trick is to pull it back just enough to reduce the pressure to 0. Pulling back further will suck in air which will cause the plastic to oxidise and harden if it left cooking in that state.

The fast forward needs to be exactly the amount rewound to keep the quantity of plastic right and avoid blobs.

The only time that the plastic is left cooking long enough to evaporate is when heating up or cooling down at the start and end of the build. I always run the extruder after it has warmed up long enough to make sure it is full and then wipe off any ooze before starting the build.

@ BodgeIT: Supermarket. It was a HG liquid drain cleaner. I called the manufacturer, they said they couldn't divulge their 'top secret formula', but did tell me this: pH was around 13-14 (which tells us the order of magnitude, but not much more). The main constituent is 26% and I assume that's the (NaOH) which is also mentioned first on the package under the heading 'contains'. It also contains 'anionogene oppervlakteactieve stoffen' (Dutch), which mean something along these lines: "non-ionogenic surface active substances", which was about 5% of the contents. I assume the rest is water to reduce concentration simply for safety reasons.

It appears that they will divulge more information to doctors, since they have a special phone number for them. I have a relative (doctor) I could ask to get this information if it's relevant. Yet I think finding a proper catalyst and skipping the use of caustic soda altogether is a good idea, unless we find nothing better. Matt Chan gave some really good suggestions what to look for:https://www.blogger.com/comment.g?blogID=12223283&postID=7144529337640299059"The optimal solution would still be an enzymatic or catalytic one though."

I think we should explore this option and use our contacts in organic chemistry!

@M.qrius: We used the block to tear it apart mechanically. I hope to conduct more experiments (I will document them on http://blog.erikdebruijn.nl ) when I get my second extruder working well with PLA. My other extruder really runs ABS pretty well.

@AKA47 You could recycle the PLA that you break (or melt) away. NaOH (lye) will break it apart chemically and you will not be able to reverse that, though. A combination of HDPE (or higher temp plastics) and PLA might be separable by heat-treatment, depending on their glass transition temperatures and melting points.

We can buy drain cleaner that is 98% sodium hydroxide here from Boots. It is obviously safe to pour down the drain and I can't imagine adding PLA changing that as PLA is not much more than sour milk and supposedly safe to put on a compost heap. The only possibility of toxicity I would think is whatever they use for the polymerising catalyst.

It's safe to pour if you do it carefully (unless your drain or sink contains aluminium or zinc, otherwise you'll get hydrogen gas). I'm just concerned about this becoming common practice of a growing population of home based manufacturers and it not being very eco-friendly. I simply heard that you should always use a mechanical drain cleaner before you use the chemical one because it's supposedly bad. We need to have some more facts about it's environmental impact. Plus it's not very safe for eyes, skin nor clothes of the person handling it. For industrial use it's not that big a deal, but for home-based use (with children around) I don't really like it, especially if we haven't explored the alternatives.

Lye can be neutralized with common household vinegar. The resulting salt, Sodium Acetate, is biologically benign. Citric acid can be used also to make the likewise harmless citrate.

"developing" models in a dilute lye solution is safe enough for adults. Minimum precautions are eye protection, gloves, and a large bottle of vinegar on hand to dilute accidental spills onto the body or especially the eyes. A dedicated eyewash station wouldn't hurt either.

Hi, I just sent email to Adrian, asking where to post contributions to extruder issues, as I think to be able to contribute in some way to the development of these extruders.Did not get answer yet, but want to keep following up what happens in regards to reversing the feeder, as I have some other ideas with possible ways to go.Just want to start some thinking with others but dont know where to post is. Any advise?Thanks for any reply.

I may have mentioned this before, but can't see my response on this page...

Reversing really solves a huge problem, but also has a problem of its own: if you 'pump' the plastic up and down the heater barrel it will transfer the heat further upward, causing congestions. I just had this with PLA again causing my bowden cable to detatch from the coupling that I had printed. It's never easy to pioneer something ;) ! Then again, they never said that it would be easy!

Hi Erik, Yes you have mentioned it before and my reply was that as it only moves back 0.75mm, never any more without first going forwards 0.75mm, the heat spread can only be 0.75mm further up so I can't see the problem. There is no cumulative effect.

If you pulled it back further, say 5mm, then I can see that could be problematical because it would move soft plastic into the cold zone where it could act like a ratchet by swelling when you push it forwards.

Maybe it isn't a problem for me because of the tapered section which is 10mm long, but without that I find it difficult to extrude from SS anyway.

Or perhaps you have a software bug where you can go back twice without first going forward again?

Or perhaps the slack in the Bowden together with the fact that the plastic is easier to pull than it is to push causes some ratcheting backwards effect.

Just trying to find an explanation for increased jamming when reversing. But I may be moving it up more than you. And in a way it is cumulative, the hotter plastic moved up. When it's moved down again it heats the plastic higher up (as I explained).

B.t.w. I did another test, PLA doesn't really dissolve in lye as well as I thought :S Not sure why it seemed to go faster before...